If you say that it is a buyer's market, you mean that it is a good time to buy a particular thing, because there is a lot of it available, so its price is low. If you say that it is a seller's market, you mean that very little of it is available, so its price is high.
[business] Don't be afraid to haggle: for the moment, it's a buyer's market.
Housing became a seller's market, and prices zoomed up.